An oscillator requires that a resonator is excited into oscillation. For oscillators without feedback, the losses (damping) associated with the oscillation means that a continuous oscillating driving signal at the frequency of oscillation or at some multiple of the operational frequency is required to maintain oscillation. However, when an oscillating signal produced by the oscillator itself is fed back to further excite the oscillator with appropriate amplification and phase control, then the oscillations can be sustained without the need for the oscillating driving signal.